Aviation and the Global Atmosphere

Aviation and the Global Atmosphere
state concentration change from a specific emission by multiplying the tracer value by the associated aircraft engine emission index (EI). Figure 3-8 shows steady-state
tracer distributions in tracer-to-air mass mixing ratio units and annually and zonally averaged fuel source used in the simulation. Table 3-4 summarizes the main results
of these simulations.
All models predict the largest perturbation at mid-latitudes in the Northern Hemisphere in the
altitude range of 10-12 km. However, the magnitude of the perturbation varies by a factor of
10, ranging from 12.6 (ECHAm 3 ) to 122 ng g -1 (GSFC-2D), reflecting differences in model
resolution and current uncertainties in modeling of global atmospheric dynamics and
turbulent diffusion. To mitigate the effects of model resolution, the tracer amount was
summed in the 8- to 16-km altitude region between 30 and 90°N (shown by the thick dashed
line in Figure 3-8). This region contains 34 (AER, ECHAm 3 ) to 61% (GSFC-2D) of the total
accumulated tracer. The absolute amount of tracer mass in this volume ranges from 2.9
(ECHAm 3 ) to 14.5 Tg (GSFC-2D). The amount of the tracer above 12 km, which serves to
diagnose the fraction of aircraft emissions transported toward the stratospheric ozone
maximum, ranges from 14 (UCI/GISS, GSFC-2D) to 45% (UMICH, AER) of each model's
global tracer amount. The global residence time of the fuel tracer, defined as the ratio of the
steady-state tracer mass to the tracer source, varies from 21 days (Tm 3 , ECHAm 3 ) to 65
days (GSFC-2D, LLNL). The lower values are similar to the global residence times
(approximately 18 days) found for air parcels uniformly released at 11 km between 20 and
60°N and followed with a trajectory model using assimilated wind fields (Schoeberl et al.,
1998). The 1/e-folding aircraft emissions lifetime of 50 days computed by Gettelman (1998)
is consistent with the results of the fuel tracer experiment described here (Danilin et al.,
1998).
The model simulation results indicate that aircraft contribute little to the sulfate mass near
the tropopause. For example, the sulfate aerosol mass density from the GSFC-2D model
(see Figure 3-8) is 0.055 mg m-3 at 10 km at 55°N, in contrast to background concentrations
of 1 to 2 mg m-3 (Yue et al., 1994). Other recent model studies (Danilin et al., 1997;
Kjellström et al., 1998) show similar results for aerosol mass; these studies further conclude
that aircraft emissions may noticeably enhance the background number and surface area
densities (SAD) of sulfate aerosol (see Tables 3-1 and 3-4) because of the smaller radii of
aircraft-produced particles.
Fuel tracer simulation also provides estimates of soot and sulfate column amounts that can
be used to calculate the direct radiative forcing of current subsonic fleet emissions (see
Chapter 6). Maximum tracer column values are located near 50 to 60°N and range from 4.1
Figure 3-9: Latitude and altitude distribution of annually averaged
increase of surface area density of sulfate aerosol (in µm2 cm-3),
calculated with AER 2-D model assuming a 1992 aircraft fuel-use
scenario, 0.4 g S/kg fuel, and 5% conversion of sulfur emissions into
new particles with a radius of 5 nm (adapted from Weisenstein et al.,
1997).
(ECHAm3 ) to 22.9 mg cm-2 (GSFC-2D). To calculate instantaneous direct radiative forcing at the top of the atmosphere from aircraft soot emissions, globally averaged
tracer column values (which are smaller than their maximum values by a factor of 3 to 4) are first multiplied by EI(soot) to obtain soot column values. For aircraft sulfur
emissions, the tracer column is scaled by EI(S), the ratio of molar mass of SO4 and S, and a 100% conversion fraction of sulfur to sulfate (see Table 3-4). For the suite
of models in Table 3-4, the upper bound for the average soot column is 0.1 ng cm-2 , with a range from 0.07 to 0.20 ng cm-2 ; for the average sulfate column the upper
bound is 2.9 ng cm-2 , with a range from 2 to 6 ng cm-2 . If photochemical oxidation lifetime and tropospheric washout rate are taken into account, a 50% conversion
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